Boo Stars Research Articles

Elemental abundance analyses with DAO spectrograms -- XXII. The B9-A3 stars Ursae Majoris, 59 Herculis, 14 Cygni and 29 Cygni

Elemental abundances of the moderately rotating B9–A3 stars λ UMa, 59 Her, 14 Cyg and 29 Cyg have been derived in a consistent manner with previous studies of this series from spectrograms obtained with Reticon and CCD detectors. The derived elemental abundances show that λ UMa is a mild Am star, while 59 Her is slightly metal-rich. Although 14 Cyg has values closer to solar than these stars, its subsolar Ca and Sc abundances indicate that it might be the hottest known hot-Am star. 29 Cyg is a metal-poor λ Boo star.

Detection of Accreting, Circumstellar Gas in a λ Bootis Star: 131 Tauri

We present IUE high-dispersion spectra of the λ Boo star, 131 Tau (HD 38545), confirming the presence of shell spectral features and also providing the first detection of accreting, circumstellar gas in the spectrum of a λ Boo star. Accreting gas can be followed in the mid-UV Fe II transitions to +80 km s-1 relative to the star. Circumstellar material can be followed to +60 km s-1 relative to the star in Zn II and is detected, albeit with low S/N, in Si II, Al II, and C I. The IUE data for 131 Tau were compared with archival spectra of other λ Boo stars with optical detections of narrow absorption features, which have previously been interpreted as signatures of circumstellar gas. We find that the circumstellar absorption in 131 Tau more closely resembles similar features that are routinely observed in β Pic and recently detected in other field A-shell stars.

Blue Stragglers in the Solar Vicinity: Newborn or Reborn

view Abstract Citations (8) References (95) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Blue Stragglers in the Solar Vicinity: Newborn or Reborn Eggen, Olin J. Abstract A selected sample of 1400 A0/1 to F2/(log Te=3.96 to 3.82) stars with accurate photometric and astrometric data are discussed on the basis of luminosities obtained from both Strömgren+Hβ and Geneva photometry. The sample contains members of the Hyades, Sirius, and HR 1614 superclusters, all of which show an appreciable age spread. The spread of 6 to 20 × 108 yr in the Hyades supercluster is reflected in a spread of about 6 to 10 × 108 yr in the Praesepe and Hyades clusters within the supercluster Strong arguments are presented for the presence of a larger age spread in the noncluster supercluster members than in the contained clusters. The age range for members of the Sirius supercluster is 2 to 6 × 108 yr and nearly all of this range is seen in the UMa nucleus of the Sirius supercluster, contained within the supercluster. The HR 1614 supercluster is near 4 × 109 yr old and may include the old disk cluster NGC 6791. About three-quarters of bright A-star sample consists of young disk stars, on the basis of their (U, V) velocities. The Am, USPC (Sct), and λ Boo stars represent about 10%, 6%, and 1%, respectively, of the total sample as well as of the young disk and old disk constituents of the sample. Although at least 7 USPC (δSct) are also Am stars, the two types, generally, are separated by about 2 × 108 yr. The rotational velocities, at various temperatures intervals, of the Am stars are about one-half those of the USPC(δSct) in the same intervals, whereas these velocities for the USPC and normal stars are nearly identical. At least 50% of the Am stars are spectroscopic binaries, half of which are double lined, whereas only 20% of the USPC are known spectroscopic binaries. The 15 λ Boo stars discussed here contain 8 young disk and 4 old disk stars, with 3 objects of unknown population because of radial velocity variations. Six young disk λ Boo stars are members of the Hyades (3) or Sirius (3) supercluster. The age of the λ Boo stars appears to be between 6 and 10 × 108 yr. The so-called "A2 IV" stars, labeled as possible peculiar objects by Abt (1994), may define the blue edge of the instability region. The USPC in the sample place the blue edge near log Te=3.904 for Mυ>+0.75 mag. Four stars classified as USPC are considerably hotter than the blue edge and are probably a different type of object. The old disk (BS) stars constitute about one-quarter of the bright A-star sample in each of the stages-normal stars, Am stars, USPC, and λ Boo stars. The spectroscopic binary frequency ranges from at least 50% for the Am stars to less than 5% for normal stars. This situation is the strongest argument for new birth as the origin of BS or, if rebirth, that whatever the required conditions they must mimic those for new birth. The strongest argument for rebirth as the origin of BS may be in the Am stars. Despite the fact that Am and USPC appear to be successive stages of evolution, separated by the blue edge of the instability region, and that at least a half-dozen USPC are also Am stars, the large difference in the apparent rotational velocity and large difference in the spectroscopic binary frequency makes such successive evolutionary stages seem unlikely. However, this very difficulty suggests the possibility that forward evolution for Am stars ends near the blue edge and coalescence or merger of the components ensues. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117567 Bibcode: 1995AJ....110..823E Keywords: STARS: EARLY-TYPE; STARS: EVOLUTION full text sources ADS | data products SIMBAD (69)

Fine Analysis of Pulsating CP Stars

AbstractWe present fine analysis results for roAp and λ Boo stars.

131 Tau: another Formula Bootis star with a shell

High signal-to-noise ratio spectra of 131 Tau indicate that this peculiar λ Boo star has a circumstellar shell similar to that of the well-known Ae shell star 17 Sex. The shell is most apparent in low-excitation Fe II and Ti II lines, but is also visible as narrow core components at Ca II H and K, and in the Balmer lines. The Hα profile of the star is partly filled in by weak emission, in addition to having a shell core. This is the first known case of a λ Boo star displaying shell characteristics in the Balmer line profiles. The presence of circumstellar material supports the recent suggestion that λ Boo stars are A-type stars which are currently accreting atmospheres from interstellar gas that has been metal-depleted by selective condensation into interstellar grains

Elemental abundance analyses with DAO spectrograms - XII. The mercury-manganese stars HR 4072A and 7775 and the metallic-lined star HR 4072B

This paper presents fine analyses of three sharp-lined stars, the mercury-manganese stars HR 4072A and 7775 and the metallic-lined star HR 4072B, consistent with previous studies of this series. HR 4072A and 7775 exhibit Pt II, Au II and Hg II lines in their spectra, while HR 7775 also shows lines of several singly ionized rare earth species. HR 4072B is very deficient in both Ca and Sc, the signature of the Am stars. For the HgMn stars, [C/Si] marginally anticorrelates with log Si/H as in normal A and λ Boo stars. The large He depletions and other departures from the solar pattern, however, argue strongly against a simple Venn-Lambert hypothesis for these stars

Carbon and Silicon in Normal a Stars and in Lambda Bootis Stars

AbstractThe anticorrelation of C and Si previously inferred from high-resolution spectroscopy of sharp-lined A stars classified as normal or mild Am is also found among the metal-deficient, more rapidly rotating λ Boo stars. The entire sample of program stars covers the range [C/Si] = − 1.14 (Sirius, mild Am star) to + 1.35 (HR 7400, λ Boo-type). Iron varies parallel with silicon. With respect to surface chemistry the λ Boo stars do not form a distinct group but rather represent the lower end in a continuous sequence of ‘metal’ abundance extending over more than 1.5 dex.We argue that differential accretion of gas and dust in late stages of star formation or during the main-sequence phase is responsible for most of the abundance variations of the lighter elements up to the iron group. Enhanced or impeded accretion of dust leads to a metal-rich or metal-poor surface composition, respectively. The heavy trace elements Sr and Ba show additional anomalies indicating that other processes like gravitational and radiative separation are also involved.

Parameters Teff and L/L⊙ for a Group of λ Bootis Candidate Stars and their Evolutionary Status

AbstractThe Teff value, according to the definition of this parameter, is computed with the procedure described; the observations are then compared with the flux distribution from Kurucz’ LTE blanketed models for various values of the metallicity parameter [Z/H].Then we derive the bolometric luminosity L/L⊙ by combining the stellar bolometric flux with the values of the trigonometric parallaxes and of the colour excess found in the literature. These luminosities are then plotted on evolutionary sequences; from the position of some λ Boo stars (ρ Vir and π1 Ori), we argue that their evolutionary stage, if interpreted as that of stars having left the main sequence, is too late to keep the λ Boo abundance anomalies.These same λ Boo stars, plotted on pre-main sequence evolutionary tracks, would indicate increasing λ Boo character when the stars are approaching the main sequence showing the strongest character in λ Boo and 29 Cyg, the two stars nearest the ZAMS.

CP Stars at High Galactic Latitude: Photometry Near the Galactic Poles ,

AbstractThe results of photometric observations of early type stars in regions around the Galactic poles are presented. The aim of these investigations was to search for CP2 and CP1 and to establish new and more precise frequencies for such objects including fainter stars.A mean frequency of 24.7% was found for the CP1 stars, 23.7% for the hot CP2, and 11.7% for the cooler ones, but with large differences between the two regions investigated.Furthermore, a substantial number of stars (20) were found to be good candidates for λ Boo stars.

The λ Boo and Related Stars

AbstractThe main properties of this class of rare objects are described, and the criteria selected for discovering new members are indicated. The abundance anomalies, resulting from the scanty analyses made up to now, are discussed together with the theoretical interpretation proposed in the framework of the modified diffusion theory and of the recent selective accretion proposal.Finally, we discuss the evolutionary stage of the λ Boo stars which, in the HR diagram, lie near, but not on the main sequence.

Pictoris and the Bootis stars: testing the accretion hypothesis

It is suggested that β Pic may be an unrecognized λ Boo star, based on the recent discussion of its evolutionary status by Paresce (1991, A&A, 247) and the proposal by Venn & Lambert (1990, ApJ, 363) linking circumstellar material with the λ Boo phenomenon. The star's metallicity, photometric indices and UV spectrum support this interpretation. If confirmed as a λ Boo star, β Pic would lend support to the accretion hypothesis of Venn & Lambert and would be the first λ Boo object recognized a priori from surrounding circumstellar material. In light of this classification, other workers' results suggesting that β Pic would be undergoing possible enrichment in metals from infalling, comet-like bodies, are discussed

Are there Normal Upper Main Sequence Stars: λ Bootis, …

Our concept of normal A stars is severely influenced by the dearth of features in the low resolution spectra of these objects that have been used for classification. The relatively small number of lines visible at survey dispersion has also led us to greatly simplify the complexity of spectroscopic patterns that can occur. The λ Boo stars were noted as a class because of the prominence of the λ4481 line of Mg II. We call attention to lines of V II and Ni II that are strong in superficially normal stars, and surprisingly weak in strong-lined stars with obvious overabundances of chromium, manganese, and iron. It is also useful to note that iron itself can be both underabundant and overabundant in CP stars. We call attention to an important new work by Venn and Lambert that develops the earlier suggestion that λ Boo stars may have formed from gas that has been separated from interstellar grains.

The chemical composition of three Lambda Bootis stars

view Abstract Citations (259) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Chemical Composition of Three Lambda Bootis Stars Venn, Kim A. ; Lambert, David L. Abstract Abundance analyses are reported for three certain members (Lambda Boo, 29 Cyg, Pi1 Ori) of the class of rapidly rotating, metal-poor A-type stars known as Lambda Bootis stars. Model atmosphere analysis of high-resolution, high signal-to-noise spectra shows that the metal deficiencies are more severe than previously reported: Fe/H = -2.0, -1.8, -1.3 for Lambda Boo, 29 Cyg, and Pi1 Ori, respectively. Other metals (Mg, Ca, Ti, and Sr) are similarly underabundant, with Na often having a smaller underabundance. C, N, O, and S have near-solar abundances. Vega is shown to be a mild Lambda Boo star. The abundance anomalies of the Lambda Boo stars resemble those found for the interstellar gas in which the metals are depleted through formation of interstellar grains. It is suggested that the Lambda Boo stars are created when circumstellar (or interstellar) gas is separated from the grains and accreted by the star. The bulk of the interstellar grains comprises a circumstellar cloud or disk that is detectable by its infrared radiation. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169334 Bibcode: 1990ApJ...363..234V Keywords: A Stars; Main Sequence Stars; Stellar Composition; Chemical Composition; Infrared Astronomy; Interstellar Gas; Metallicity; Stellar Mass Accretion; Stellar Rotation; Astrophysics; STARS: ABUNDANCES; STARS: ACCRETION; STARS: PECULIAR A full text sources ADS | data products SIMBAD (4)

Spectroscopic tests of photoelectric stellar classification of abnormal stars

view Abstract Citations (57) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopic tests of photoelectric stellar classification of abnormal stars. Abt, H. A. Abstract Spectroscopic classification is obtained for 169 northern A5-G0 stars predicted by Olsen (1979, 1980) to have abnormal spectra on the basis of Stromgren four-color photometry. The success in identifying reddened early type stars was nearly 100 percent, for Am and early type weak lined stars about 75 percent, for stars above main sequence about 50 percent, for composite spectra about 25 percent, and for Ap and Lambda Boo stars 0 percent. Thus photoelectric photometry is a successful first step in discovering stars of the more extreme spectroscopic abnormalities. Publication: The Astrophysical Journal Pub Date: October 1984 DOI: 10.1086/162498 Bibcode: 1984ApJ...285..247A Keywords: A Stars; Electrophotometry; Metallic Stars; Peculiar Stars; Stellar Spectra; Stellar Spectrophotometry; Early Stars; Interstellar Extinction; Line Spectra; Northern Sky; Astronomy full text sources ADS | data products SIMBAD (170)